Signaling system



. Feb. 25, 1930.

H. NYQUIST SIGNALING SYSTEM Filed Feb. ll. 1928 3 Sheets-Sheet l 3 Sheets-Sheet 3 LPF 104 H. NYQUIST SIGNALING SYSTEM Filed Feb. 11, 1928 Feb. 25, 1930.

INVENTOR BY EtQ/guw ATTORNEY Patented Feb. 25, 1930 UNITED STATES PATENT OFFICE HARRY N'YQ-UIST, 0F .MILLBURN, NEW JERSEY, ASSIGNO T0 AMERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION 0F NEW YORK SIGNALING SYSTEM Application mea February 11, 192s. serial Nu. 253,754.

It is an object of my` invention to utilize a transmission line and other facilities `to the best advantage for signaling. Another obj ect of my invention is to s'end a sequence of sig- ,3 nal currents with a range of frequency as narrow as practicable ;0r having given a certain available range for such components, to send at as high a speed as practicable within that range. Another object of my invention is to transmit signal currents effectively on a carrier .current with only one side-band of frequencies, as will be explainedherein'after. Other objects of my invention are to proj `vide a method and the proper apparatus for,

* sending signal currents on one side-band per signal channel and making proper compensation for unequal attenuation and phase delay in the jsystem employed. Still another object of'my invention is to provide-for overcoming the distortion due to the filter employed when transmitting signal currents on only one side-band of frequencies. All these objects and other objects of my invention will become apparent on consideration of a o5 limited number of examples of practice according to the invention which I have chosen for disclosure in this specification. Itvwill be understood that the following disclosure relates principally -tothese particular examples of practice of the invention, and that its scope is intended to be indicated bythe appended claims.

This application is in part a continuation ofmy application, Serial Number 198,283, V, l filed June 11, 1927, title Signaling system. v

- Referring to the drawings, Figure 1 is a diagram of apparatus at thesending end of-a v telegraphsignaling system; Fig. 2 is a diagram of the apparatus at the corresponding receiving end; Fig. 3 is a diagram indicating the relation of various frequencies and frequency ranges that will be mentioned;'Fig.

l1 is a characteristic diagram for certain band pass filters that will be mentioned; Fig.4 5 is a diagram of a telephotograph system lembodying my invention and employing a balanced demodulator at the .receiving end; Fig.

6 is a diagram of such a system without the balanced demodulator at the receiving end; l Fig. 7 is a diagram of a system that may bev 50 used alternatively for telegraphy or telephotography and employing a balanced demodulator for which carrier current is separated out at the receiving end; and Fig. 8

is a set of curves based on the characteristics of certain filters of Fig. 7.

Referring to Fig. 1, thetelegraph sender 14'generates signal waves of direct current corresponding to the signal elements and applies them to the input of the low-pass filter 15 which passes only those frequencies between zero and fm. The cut-off frequency fm is chosen atA as low a value as practicable to leave the wave coming from the low-pass filter 15 definite enough to identi the'in- 65 tended signal. This signal of requency components between zero and fm is applied through the conductors 16 in the modulator 17 to modulate a carrierV current fc from the generator 18. It is assumed that f f,.

Accordingly, as is well known, the conductors 19 from t'he modulator 17 will carry currents lying in frequency between )cc-fm and fc-lfm, andthese vwill be applied to the bandpassfilter 13, whose pass range is approximatelyv from fc-fm to fc, and accordingly only the components of Afrequency within this'pass range will get from the band-pass filter 13 through the conductors 12 to the line 20.

Similarly, other senders will put their respective side-bands ontheline 20 through other band-pass filters, such as 13.

At thereceiving end the composite signalingvcurrents on the line 20 are separated from 85 the ,branch conductors 21 by the band-pass filters 22, 22', etc., which correspond respectively to 13, 13', etc. at the sending end. Thus, the band-pass filter 22 passes only the` vside-band from fc-fm tojccarrying signals from the sender 14. These currents through the band-pass filter 22 are then put through the phase corrector 23, which equalizes delays on the line andA through the filters.

Then the currents from the phase corrector 23 lgo over the conductors 24 to the input transformer 25 for the balanced homodyne detector comprising the two vacuum tubes 26 and 27.

One of the generators 18" at the sending end sends current of a frequency fo (fo is different from frequencies lying in the pass.

bands of the other filters 13, 13', etc.)

through an appropriate band-pass filter 13,`

nected by means of a shaft to have definiteA frequency and phaserelations, and likewise, other generators, such as 28', at the receiving end are similarly connected. It is thus nec- Iessary to have only one synchronizing channel such as denoted by the-band-pass filters 13" and 22 for several sending channels, just as -in the case of phase discrimination carrier telegraphy.

As will be explained hereinafter, the band filters 13 and 22 are made with special design and there may be other provisions so that the current from the detector 26-27 through the resistance 31 and lowpass filter 32 to the receiver 33 is substantially an` undistorted copy of the current in the conductors 16 from the low-pass filter 15 at the sending end.

Asis well known, any signaling current containing frequency components from zero to fm may be used to modulate a carrier cur-v rent of frequency fc where fc fm, and in this case the modulated current vwill occupy a band widthv from fc-fm to fc-ifm, that is, a width of2fm. In carrier telephony it is possible to transmit only half of this band and obtain intelligible speech currents after detection at the receiving end. This is readily done because in-the original signal, current components of value close to zero are not essential and hence no harm is done if the apparatus distorts currents of frequency near the. carrier, as is likely to be the case in a filter which cuts off half'of the'band mentioned. p a This transmission of only one side-ban enables one either to increase the number of available -channels when fm is fixed, or to improve the quality of the telephone transmission by including higher frequency components when the number of channels is fixed.

In carrier telegraphy the transmission of only one side-band would likewise make possible an increase in the number of channels or in the speed of signaling, but this has been distortion, and (2) that very low speech frequencies are not essential to good understanding. However, in the case of telegraph signaling it is important that after detection the original signal components of frequency from zero to fm. shall be reproduced in substantially correct amplitude and phase relationships if the wave shape is to be unaltered. This object is accomplished by the system shown in Figs. 1 and 2, subject to features of design that will be pointed out presently.

For a signal wave to be transmitted, the output current in the conductors 16 of Fig. 1 will be a certain function of time t. Assuming that this signal wave is repeated at long intervals it may be'accurately represented by a Fourier series; and in practice this representation will be sufliciently accurate for a single signal wave not repeated. Accordingly, we have the equation for such a signal element,

, m j@ E (An cos npt-l-Bn sin apt) 'n I l m l M1. sin @N+ e.) (1) n 0 j where p=21r times the assumed frequency of the long interval M xAzn B2n except .that lll0 and l com rises the an e of `fre uencies froml As a'function vof time t, the current from the generator 18 is given by the expression sin met, and this is to be multiplied by f(t) o'f Equation (l) to get the output from modu-l lator 17; the result is m f(t) sin wt= llln sin (npt+9) sin wct As n takes all integer valuesfrom zero to m, it will be seen from the foregoing expression that the output current from modulator 17 fc-fmto ffl-fm. This range is cut down by the band-pass filter 13 to approximately the range from fc-fm to f and transmitted on the line 2O as described heretofore. I

For the present it. will be convenient to make two assumptions: (1) that the filters 13 and 22 cut ofi' sharply, and (2) that their cut-off frequencies-are Where w mk w, and wk=kp=mp, and wh=hp. These values are indicated in Fig. 3. These two assumptions will be considered later. Subject to these assumptions, the current (call it I (t)) in the output from phase corrector ,23 may readily be deduced from Equation '(2) as having the following value:

For convenience of discussion, let the three summations in the last member of the foregoing equation be represented respectively as a1, a2 and a3, so that By substitution from Equation (1), al becomes HU) sin wat, which is in phase with the carrier current, since it contains the factor sin met. The envelope of this component than the signal at the sending end and its presence is objectionable. It will be eliminated by making w=0, that is, by making the band-pass filters 13 and 22 cut off at fc at the upper ends oftheir transmitting' ranges respectively, provided as is here assumed, that such cut-off is sharp.

The remaining component as contains the factor cos wot, and is therefore in quadrature with the carrier current. The phasel adjuster 29 is adjusted so that the electromotive forces applied through transformer 30 are in phase with the received carrier cur- M,..cos wat cos (npt+9) (3) [214,608 at eos (apta-9,.) -M sin ai sin (npf+e)] rent as represented in the term al of Equation (3a), and therefore in quadrature with the current component represented by the term 03. Since the components in the output from the homodyne detector 26-27 are to be found only among the components of the product of the'input currents'through transformers 25 and 80, land since currents in quadrature give zero average product, it follows that the output from filter 32 will comprisel no components due to the term 03. v

Of the two assumptions that were noted I above between Equations (2) and (3), we

have made the seconddeiinite by further assuming =O. As to the first of these two When the products of modulation in de? vice 17, as given by Equation (2), have been actually ltransmitted to the input of transformer 25 at the vreceiving end with varying attenuation and varying phase shift for .the

components of vario-us frequencies,they may be represented by the following expression,

which is obtained by introducing amplitude factors m and te, and phase shift terms and pn in Equation (2) Note that when n=0the two angles for the two cosine terms expressed in Equation (5) become of the saine angular velocity though different in phase, that is, both involve the same frequency. Therefore, the factors `r -and uo will be equal in this case, and accordingly, the two angles4 O and p0 will be equal.'

Let 'the current applied from source 28 through the phase adjuster 29 in the transformer 30 have the value 4 sin (wot-Q50). Accordingly, the components of the output current from detector 26-27 will be found among the terms in lthe product of this eurrentfrom source 28 by the current of Equation (5). Making this multiplication, and.

, E M, [cos npT sin (npt+9) 4 sin npT cos (apt-FGM (4) making the transformation indicated by the trigonometric identity.

2 sin a cos b=sin(alb) +sin(a-b), and neglecting terms corresponding to sin (a+ b)- for the reason that the corresponding currents will be stopped by filter 32, we get, as the output from that filter,

' with the respective terms, giving 0,. sin [asin cfa) man tio-pai 8) and i 0,. cos 1.. cos (glam-gto) -l-un cos (asf-fm) (9) These Equations (8) and. (9) are obtained by equating coefficients of sin '(npt+9) and cos (apta-9,.) in the right hand numbers of Y Equations (6) and (7).

The solution of Equations (8)v and (9) gives the results f the actual system shown in Figs. 1 .and 2 will be with uniform attenuation and undistorted and subject'` only to a vconstant delay for all frequencies if such values can be given to n., 11,., qs.. and p.. as to make 0,. constant in Equation (l0) and to make ,8n proportional to frequency in Equation (11). In other words, it should be the object of design to make the transmitting systemy from the lowpass filter 15 to the receiver 33 such that =npT and 0,. is constant, and this would be accomplished by making n, um gl, and p have proper values according to Equations (1l) and (10). Thereupon Equation (6) for the current actually received will become the same as Equation (4) for the current that it is' desired to receive for undistorted transmission.

Inspection of Equations (10) and (11) shows that there exist an infinite number of combinations of values of n, um 4. and p which will exactly or approximately fulll the requirements that c,L shall be constant and thatA shall be proportional to frequency. Further inspection of Equations (10) and (11) shows that a convenient com bination of values will be Obtained by making the band pass filters 13 and 22 so that their upper cut-off frequency will be near the carrier frequency, and so that 1lu=a constant, and equalizing the delay so that 0=p-0=npT. This makes the angle zero for the cosine in Equation (10) and this equation then reduces to c=r+u. Also, in Equation (11) these same-conditions reducev it to =hpT.

Furtherto illustrate the foregoing principles, make the reasonable assumption that the band-pass filters 13 and 22- of Figs. 1 and 2 have the actuall transfer admittance given by the characteristic curves 4l and 42' in Fig. 4. Also, assume that their delay as a function of frequency is in accordance with the curve '46. These assumed characteristics are in accordance-with easily realizable examples of practice. The delay indicated by the curve 46 is so smooth that phase equalizers can be omitted when the carrier frequency fc is located at 1048.5 cycles, and the location at this point gives very nearly conl stant values for rlu, the us being taken at frequencies equally spaced above fc and the corresponding rs being taken at equally spaced intervals below, as shown by the dots on` curve 41. In similar manner, the dots on curve 42 indicate values of 4, below 1048.5 cycles and pn above 1048.5 cycles.'

In short, having the plots 41 andv 42 in Fig.

4, we arbitrarily seek tolocate fc at a point that makes nfl-26 as nearly constant as practicable. This We do after noticing that 'the phase angle varies linearly at or near the point chosen; if it did not do lso we would first make suitable correction for` phase by means of device 23` in order that it would .Assuming that fv.=80, that is, that the essential frequency range for components of the initial signal are 'from zero to 80, this implies transmitting the side-band from '1048.5 down to 968.5. In all this range (and beyond at the right) the phase angle is nearly proportional to frequency, as desired. As explained in the first part of the specification, it wouldbe satisfactory if the modulus of admittance had the ideal form-indicated by the dotted line 43. The effect of departure from this form at 44 is overcome by the relationships that have already been dis'- cussed in connection with Equations (6) to.

(11). The departure at 45 is not serious; indeed, it is negligible. For proper interpre tation of the signal itfhas been assumed that signal component frequencies above 80 may be entirely neglected, which means that in' 45 may be minimized bysuitable design of the filters 13 and 22.

4and 22".

By throwing. the switchesf), 8, 5 and 4 in Figs. 1 and 2, transmission of signals may.

be effected without sending the special current of frequency f., through the filters 13 ing to this suggestion, it becomes necessary to send a considerable component ofcarrie-r" frequency'as part of the side band transmitted through the band pass filters 13 and 22. In other words, it becomes necessary that there shall always be a considerable component of current atthe frequency corresponding to fc=1048.5 in Fig. 4. In this case the received current will be detected in the ordinary way by the detector 7 of Fig. 2,

whose output will go through the low pass filter 6k to telegraph receiver 33. Referring ltofthe foregoing discussion following Equation (3a), it will be seen that the objectionable term of Equation (3) will be present -when the receiving path through detector '7; is employed; but under certain practicable conditions of operation, it will no t be great enough seriously to impair the recelved signals. That is, the loss of quality or speed With the switches shifted accordor both due to the presenceof this term may not be great enough toy offset the gain by the use of simpler receiving apparatus and the elimination of a special channel for carrier current.

In either case, that isV with switches 9, 8, 5 and 4as shown or all shifted, the special design of filters 13 and 22 will be advantageous according to the foregoing discussion in connection with Fig. 4.

Referring to the'diagram in Fig. 5, this shows a. single side-band picture transmitting system arranged for the practice of my invention. The glass drum 51 is mounted coaxially .with the shaft 52, which hasl screwthreaded engagement with the support 53 so that as the drum 5l rotates, it is traversed slowly along its length. The synchronous motor 54 drives the drum through gears and a sliding spline connection. Light from the lamp 55 is `focussed by the lens 56 on the photoelectric cell 50 within the glass' drum 51. The picture to betransmitted, in the form of a semi-transparent film, is wrapped about the glass drum 51 and the narrow area of the film through which 4the light shines describes a helical course with close turns -current goes through the band lter which cuts off the lowerside band according to the principles heretofore disclosed in connection with the band filter 13 of Fig. 1. This lower side band of modulated carrier current is then put on the line for transmission to the receiving station.-

The alternating current generator 61 delivers current of 300 cycles per second to drive the synchronous motor 54 mentioned heretofore. Also, this 30G-cycle current is passed through the band filter 62 and superposed-on the line 65 along with the modulated carrier current that comes through the band filter 60; Still another portion of this 30G-cycle current'goes through the distorter 63 whose output is accordingly a current rich in harmonics.` The band pass filter 64 passes the harmonic of frequency 2700 cycles per second, which is applied as the carrier current to be modulated by the .varying picture current `in the balanced modulator 58-59.

At the receiving end, the band pass filter 66 draws ofi' the current of BOO-cycle frequency and sends it through the amplifier 67 to drive the synchronous motor 68, thereby l sending end. Light from the lamp 72 is clirected by lenses t lrough a variable orifice in the light valve 73 and on a spot of the drum 71, and this spot describes a helical course with close turns relatively to the drum 71.

l Some of the 300-cycle current from the band pass filter 66 goes to the distorter 74 whose output is connected with the band pass filter 75 which passes only the harmonic of frequency 2700 cycles per second. This current of carrier frequency 2700 cycles per second goes through the adjustable phase shifterl 76 to a pair of input terminals of the modulator 77.

The lower side band of current from 2700 cycles down comes from the line 65 through the band pass filtero78 and thence through the phase corrector 79. The filter 78 corresponds with ,the filter 22 in Fig. 1, and the phase corrector 79 corresponds with the phase corrector 23 1n Flg. 1. This lower slde band of current, properly corrected in phase,

is then combined in the modulator 77fwith the current of 2700 cycle frequency so as to effect demodulation, and the demodulated current goesy to the light valve 73 and controls its opening in accordance with the amount of `51 is a negative, the film on the drum 71 will be a positive, and vice versa.

Comparing the system of Fig. 5 with that of Figs. 1 and 2 (with the switches 9, 8, 5 and f 4 as shown), it will be seen that in both cases the signaling current is applied to modulate a carrier current, and that only the lower side band of modulated carrier current is transmitted. A balanced demodulator is em-l ployed at the receiving end in both cases to eliminate the effect of currents corresponding t the term 08 of Equation (3a) also in both cases the filters that cut off the lower side band are of special design, as pointed out in connection with Fig. 4. namely, the filters 13 and 22 in Fig. 1, and the filters 60 and 78 in Fig. 5. Also, in both cases, a phase corrector (23 in Fig. 1, and 79 in Fig. 5) is employed,if needed, to get the phase anglev characteristic of approximately uniform slope, as shown in Fig. 4.

In Figs. 1 and 2. and likewise in Fig. 5, a

special current of -frequency fo is trans-- mitted over the line for synchronizing and forl derivingv the carrier current at the receiving end, but in the system shown in Fig.. 6 there 1s no such transmission.` Here, the

synchronous motor 54 at the sending end rotates the glass drum 51 as in Fig. 5, and, in a similar. manner to Fig. ,5, the current from the photoelectric cell -is amplified at 57 and applied in the modulator 58 to modulate carrier current of any suitable frequency .from the generator 61. Only the lower side band of the output from the modulator 58 goes through the specially constructed filters .60 and 78 and over the line 65 between them. At the receiving end, the received current goes through the phase corrector 79 and amplitier 93 to the light valve 7 3. This light valve 73 is ofdifi'erent construction from the light valve 73 in F ig, 5. The valve 73 isoperated by the demodulated current correspondingto the current through the photoelectric cell 50; but the light valve 7 3 is operated by the modulated 'carrier current so that it opens and closes at the frequency of the received current and the extent of the opening corresponds to the amplitude, that is, corresponds to the magnitude of the modulating current at the lsending end, which is the current in the circuit of the light valve 50.

The drum 71 at the receiving end is rotated by the synchronous motor 68 in a manner similar to that described for Fig. 5. It remains to describe how the motor 68 at the receiving end is kept substantially in syn-l ceiving ends, the motors 54 and 68. Referring to the sending end, the box 82 contains a tuning fork 83 whose vibrations are sustained by an appropriate current in the coil 86. This current comes through the phase adjuster 85 from the three-electrode vacuum u Vtube oscillator 84 whose frequency is determined by the currents in the coil 87 in the input circuit. Accordingly, the currents delivered from the oscillator 84 to the amplifier 88 will be, of the natural frequency of the tuning fork 83. The currents from the amplifier 88 go through the repeating coil 89 to drive the synchronous motor 54.

The synchronous motor 54 drives the glass drum 51, as already pointed out, and, in addition, .it drives a' cumulative indicator 8l, in the nature of a revolution counter. comparing the indicator 81 with an independent standard clock 81', it can be noticed whether the motor 54 is running fast or slow.

A small heating coil is provided at 90 within the box 82, and by adjustment of the associated rheostat, the temperaturewithin the box 82 can be changed a little so as slightly to vary the rate of the fork 83 and correct luf.

' any tendency of the indicator 81 to get out of step with clock 81.

At the receiving end, there is an independent standard clock 92, andthe synchronous motor 68 drives a cumulative indicator or revolution counter 92 which can be compared with the clock 92 in the same way that comparison is made at the sending end between 81 and 81. Means of adjustment are comprised within the apparatus 91 the same as 'within the apparatus'91 at the sending end.

Accordingly, synchronism is effected between the sending and receiving ends by keeping the apparatus at those two ends in synchronism with standard clocks, which, of course, are regulated-in the usual way either directly orl indirectly by the earths rotation as determined by the meridian passage of an observed star.

In Fig. 7, a system is shown adapted altei-natively for transmitting telegraph signals or pictures. lIn this system, only the lower side band is transmitted, but this comprises, at its upper end, an appreciable comjso p'onent of carrier frequency, and this component 1s utilized. at the receivlng en'd for synchronization and as a source of carrierfrequency for demodulation. When sending pictures, the switches 100 at the sending end, and 101 at the receiving end,'will be thrown down, as in the drawing, and the switch '102, at the receiving end, will be' closed. In this case, the sending end will be -very similar to that 4shown in Fig. 5, except that the carrier current of 2700 cycles per second is generated directly in the generator 61 and is applied not only to the modulator 58 but also t0 the synchronous motor 54.

At the receiving end, the cur ent all. goes through the special band pass lter 78 that passessubstantially only the lower side band. Onthe output side of this filter, there are v multiple branches going to the phase corrector 79 andfto another band pass; filter 66. The transfer admittance characteristic for the band pass filters 60 and 78 is shown at X in Fig. 8. It will be seen that the 'righthand part of this characteristic has the same relationto the carrier frequency vof 2700 cycles per second that the right-hand part of the characteristic 41 for transfer admittance in Fig. 4 has to the carrier frequency in that case, which is 1048.5 cycles.per second.

The band pass filter 66 is designed so thatV its transfer admittance characteristic Y in Fig. 8 is symmetrical to the characteristic X with respect to the ordinate for 2700cyc`les; that is, XPX is the mirror imageof'YPY. Accordingly, the attenuation characteristic for the current through the band filters 60, 78 and 66, that is, the current in the output of 66 is symmetrically attenuated. on both sides of the ordinate for `2700 cycles in Fig. 8.

This current in the output of band filter 66,

will therefore have lthe basic lfrequency of 2700 cycles per second, but will vary somewhat in amplitude. It goes to the input of l the current limiting device 96, which may be a thermionic vacuum tube operated on the saturation partof its current-voltage characteristic. This gives an output current of thevbasic carrier frequency and "of substantially uniform amplitude. This currentgoes through the phase shifter 76 to the receiving modulator 77 and also through the amplifier 67 to drive the synchronous motor When the system is to be used for telegraph signaling instead of picture transmission, the switch 102 is opened, and the switches and 101 are thrown up. The

alternating currentgenerator 97 generates a y current of carrier frequency, and its amplitude is changed back and forth between two stages, one for marking and the other for spacing by means of the key- 99 controlling a short circuit or shunt around the'series re and applied tothe modulator 77. The outputfrom the modulator 77 then goes through a low pass filter 103 to the telegraph receiver 104 the same as to the receiver 38 in Fig. 2.

I claim: i 1. The method of signaling on an approximate single side-band which consists in cutting that vside-band off somewhat gradually near the carrier frequency, with phase shift increments proportional to frequency incre' ,ments near the carrier frequency and wlth amplitude factors equally above and below the carrier'frequency that add in pair-Stoa constant value, wherebythe distortion that mightfbecaused by departure from a sharp cut-off is obviated. v

, 2. The method of signaling which consists in generating a vsignal wave of current, applyingit to modulate a carrier current, transmitting approximately only one side-band of suchl modulated current, making the cut-off -gradual on both sides of the carrier frequency, varying the phase shift uniformly near the carrier frequency and varying the amplitude oppositely and symmetrically about Vthe carrier frequency as a mean value,-

and at thereceivi-ng end detecting this current with balanced detector elements'and a llocally applied current in phase with desired arcaica components, whereby undesirable` components in quadrature are eliminated.

3. The method of cutting off a part 0f a frequency range of modulated carrier currents which consists in making the cut-off gradual on both sides of the carrier fre quency, and varying the phase shift uniformat thecarrier frequency, and compensating for the approximate character of the cut-o by varying the phase shift increments* proportionally with frequency increments near the carrier. frequency, and by varying the amplitude factors in the same `range so that any pair of such factors equally spaced above and below the carrier frequency shall add to a constant value.

5. The method of signaling on an approximate single side-band which consists in cutting the side-band off somewhat gradually on-both sides of the carrier frequency, and receiving with balanced detection to eliminate undesired components in quadrature with those desired, and varying the phase shift and amplitude factor at and near' the carrier .frequency to compensate for the gradual cut-off as compared with an abrupt cutoff.

6. The method of signaling on an approximate single side-band of carrier current in cases where current components offrequency close to the carrier frequency are essential,

- which consists in compensating components near the carrier frequency above and below it to obviate distortion due to the necessarily gradual cut-off for the side-band transi mitted.

7 The method of compensating for gradualness of cut-off by a filter, which consists in varying the component currents above and below a suitable mean cut-od Value of frequency, so that components above and below" will combine to effect the desired compensation.

8. In the method oftransmitting signals on a single side-band of carrier current, th'e procedure which consists in making the cutoff gradual on both sides ofthe carrier frequency and varying the phase shift uniformly near the carrier frequency and varying the amplitude oppositely and symmetrically about the carrier frequency as a mean value,

and in homodyne receiving on a balanced detector whereby received components of cur'- rent in quadrature to those deslred are eliminated.

v rent thereby, a line, a filter that will pass to the line approximately only one side-band of the modulated current, said filter being adj usted so that it cuts off somewhat gradually at and near the carrier frequency, and with phase shift increments proportional to frequency increments in this range, and with amplitude 'factors that'add to a constant value equally spaced above and below the carrier frequency, and a balanced homodyne detector for receiving the transmitted sideband.

10. In combination, means to generate a signal current and to modulate a carrier current thereby, a line, a liltervto pass approximately only one side-band of the modulated current and put it on the line, and a balanced homodyne detector to receive the current from the line, the said filter being designed to cut off at or near the carrier frequency, with phase shift increments proportional to frequency increments in the neighboring frequency range and with amplitude factors that add to a constant value at equal frequency intervals in pairs above and -below the carrier frequency.

11. The method of utilizing a frequency range effectively for multiplex signaling which consists in applying the currents of the respective signal channels to modulate carrier currents, cutting ofi' and transmitting only one side-band foreach signal channel, and compensating the currents near the cutoff frequency for each channel to obviate distortion that might be caused by the necessary gradualness of the cut-ofi".

12. The inethod of signaling with a band of frequencies requiring the presence of low frequency components, which consists in applying 'said band to modulate a carrier current of higher frequency, thus producing upper and lower side bands with essential frequencies close to the carrier frequency, and cutting off and transmitting one such side band with gradual cut-ofinear the carrier frequency, with phase shift increments proportional to frequency increments near the carrier frequency and with amplitude factors equally above and below the carrier frequency that add in pairs to a constant value, whereby the distortion that might be caused by departure from a sharp cut-olf is obviated.

13. The method of cutting ofi' one of two side bands adjacent to a carrier frequency, which consists in varying the modulus of transfer admittance symmetrically as a function of frequency above and below the carrier frequency and varying the angle of phase shift uniformly as a function of frequency above and below the carrier frequency.' l

14. The method of cutting of one of two side bands adjacent to a carrier frequency,l which consists in varying the transfer admittance gradually as a function of frequency near the carrier frequency to compensate frequency.

-values thereofabove and below the carrier ing frequency range on both sides of the carrier frequency so as to compensate for the` said necessar lack of sharpness.

,17. In com ination, means to generate and Lmodulate a carrier current, andza filter to ut-oif a side band-"of the modulated current,

the/cut-oi being necessarily somewhat gradual, and the carrier frequency being adjusted latan intermediate value in the range of amplitude factorsgin the region of said grad; ual cut-off sothat c., shall Abe constant in Equation ('10) and shall be to frequency in `Equation (11).

In testimony whereof,l I .have signed my name 'to this specification this 10th day of February, 1928. f A

HARRY NYQUIST.`

proportional gradual cut-olf to effect compensation there- 18. In combination, means to generate a low frequency, means to generate a carrier current of a certain frequency, means to modulate the carrier current by the signal current thus producing .two side bands with essential frequencies close to the carrier frev quency, and means to cut off one sideband .with necessary gradualness, the frequency of the said carrier current being adjusted to an intermediate frequency value in the range of gradual cut-0H to compensate for the departure from abruptness of cut-olf.

19. The method of signaling with a band of frequencies requiring the .presence of low frequency components, which cbnsists in applying said band to modulate a carrier current of higher frequency, thus producing upper and lower side bands with essential fre-v quencies'close t0 the carrier frequency, and

cutting 'olf and-transmitting one such side' band with gradual cut-oif near the carrier' frequency, with phase shifts and amplitude factors at various frequencies near the carrier frequency so related as'to make the phase f signal current with essential components of shifts proportional to frequency and to make' I the amplitude factors constant for the low frequency ran e.

` 20. The met od of signaling with a band i of frequencies requiring, the presence' of low 'frequency components, which consists in applyin'g's'aid band to modulate a carrier current:-; ;of higher frequency, thus producing upper-,and ower side bands with essential frequencies close tothe carrier frequency, and cutting olf and transmitting one "such slde band with gradual cut-0E near the carv'rier frequency, demodu'lating after the transmission, and'making the phase shifts and iso: 

